1. Field of the Invention
The invention relates in general to gas-insulated transmission lines and in particular to gas-insulated transmission lines having insulators designed to reduce the electric field at the critical insulator conductor interface region.
2. Description of the Prior Art
A common insulator used for supporting the inner high voltage conductor centrally within the outer conductor of a compressed gas insulated transmission line is the cast insulator which is cast in place around the inner conductor or onto a thin metal sleeve which is secured to the inner conductor. A material, such as epoxy, is usually selected which has a coefficient of expansion similar to the metal selected for the inner conductor so as to minimize the possibility of voids being formed at the critical interface where the insulator meets the conductor. This is because such voids are subjected to high electrical fields at the critical insulator conductor interface region, which can lead to ionization within the voids, flashover and a reduced life expectancy for the insulator. This high electrical field at this critical region approaches a value equal to the product of the field at the inner conductor in the gas and the dielectric constant of the insulator. From this it can be seen that selection of an insulator material with a low dielectric constant would have advantages. In the prior art however, epoxy, which has a relatively high dielectric constant of typically 4 to 5, is generally used. The voids at the critical interface region are usually avoided during the casting provided proper procedures such as vacuum casting around the inner conductor are utilized during the manufacture of the insulators. Other materials with lower dielectric constants such as polypropylene or polysulfone with dielectric constants in the range of approximately 2 to 3 have not been successfully molded or cast around typical inner conductors without creating voids since they have much higher coefficients of thermal expansion. However, insulators made from these low dielectric constant materials have been produced by injection molding methods, for example, and have been used successfully as insulators in gas-insulated equipment transmission lines provided the insulator utilizes a conducting layer on the insulator at the conductor interface and/or has shielding rings at the conductor. This has been used with insulators of thick cross section. Thin cross section insulators have been used without the metallizing where an I cross section has been used to maintain a low field at the interface. These thin insulators (less than 1/4 inch thick) have limited mechanical properties, and it would be a significant advantage to be able to make them of thicker cross section, which usually then requires metallizing the interface surface of the insulator.
Metallizing the insulator interface surface is expensive and the matallization is subject to flaking, the voids then formed being subject to the same or greater increased fields as at the critical conductor insulator interface. Accordingly, it would be advantageous to have an insulator structure that can be made by the inexpensive methods of production such as injection molding wherein metallizing of surfaces is dispensed with and the finished insulator product by means of suitable design would have a flashover voltage at least as high as the present epoxy systems.